Battery packs have already been used in a variety of portable apparatuses such as a mobile telephone, a digital still camera, a portable game machine, a notebook type personal computer, and an electric-powered tool. Further, at present, the battery packs are being used not only in these, but also in the fields in which higher output or higher capacity is necessary, such as an electric power-assisted bicycle, an electric vehicle, and a household electrical storage device.
As a secondary battery cell incorporated into the battery pack, a lithium-ion secondary battery cell is present as one of those which are currently most vigorously used. The lithium-ion secondary battery cell can be repeatedly used through charging and is being used in a very wide range due to a number of features such as a high voltage output, high energy density, less self-discharge, and long service life. Further, in order to respond to demands for apparatus with higher output and higher capacity, cases are also being increased where a number of secondary battery cells (single cells) are connected in series or in parallel, thereby being used in the form of an assembled battery. Although such use has a big advantage, since a very large amount of energy is treated, more careful handling than in the past usage is necessary.
In a case where deterioration of the secondary battery cell constituting the assembled battery advances, so that an internal short-circuit occurs, excessive current flows in from all other secondary battery cells parallel-connected to the secondary battery cell. If such a phenomenon occurs, there is concern that a problem may arise in which not only the internal short-circuited secondary battery cell, but also the other secondary battery cells which are discharged rapidly produce heat.
As means for solving such a problem, in a technique disclosed in Japanese Unexamined Patent Application Publication No. 2008-135342, by disposing a current limiting element such as a constant current diode at a portion where the secondary battery cells are connected in parallel, generation of excessive electric current is suppressed. However, in the technique disclosed in the above Japanese unexamined patent application publication, there are problems in which not only the disposition of the secondary battery cell is limited by the shape of the current limiting element, but also the conductive member which interconnects the secondary battery cells is complicated and further, production costs increase.